1. Field of the Invention
The present invention relates to an apparatus for measuring the resistance value of a resistor in function of parameters of a current signal which flows through the resistor. More particularly, this invention concerns--although not exclusively--a measuring apparatus especially well suited for measuring the resistance values of a resistor in function of the amplitude of a direct current signal flowing through the resistor or in function of the frequency and the amplitude of an alternating current signal flowing through the resistor.
2. Description of the Prior Art
In particular, the resistors measured by the measuring apparatus of this invention are that of relay contact blades inserted in known telephone exchanges. It has been noticed that the impedance value of a relay contact element and its insertion loss increase when the amplitude of the alternating current signal applied to the relay and having a predetermined frequency decreases. Complementarily, similar variations of the impedance value and the insertion loss of the relay contact element occured when the frequency of the alternating current signal having a predetermined amplitude fluctuates.
Known resistor measuring apparatus are only adapted for measuring resistors in function of one parameter of the current signal which flows through the resistor to be measured. These apparatus differ principally in their operation method:
either using a direct current signal by means of a measuring bridge, a d.c. supply and a d.c. microvoltmeter; PA1 or using an alternating current signal by means of a measuring bridge, an a.c. supply producing an alternating current signal having a constant amplitude and an a.c. microvoltmeter. PA1 first switchable generating means for generating direct current signals having predetermined amplitudes; PA1 first switching means for controlling said first switchable generating means and thereby selecting a direct current signal having a predetermined amplitude; PA1 second switchable generating means for generating alternating voltage signals having predetermined frequencies and a constant voltage amplitude; PA1 second switching means for controlling said second switchable generating means and thereby selecting an alternating voltage signal having a predetermined frequency and said constant voltage amplitude; PA1 switchable means for selectively attenuating said selected alternating voltage signal having a predetermined frequency and said constant voltage amplitude; PA1 third switching means for controlling said switchable attenuating means and thereby selecting an alternating voltage signal having a predetermined frequency and a predetermined voltage amplitude; PA1 voltage to current converter means for converting said alternating voltage signal having a predetermined frequency and a predetermined voltage amplitude to an alternating current signal having a predetermined frequency and a predetermined current amplitude; PA1 means for selectively applying to said resistor, the resistance of which is to be measured, said direct current signal having a predetermined amplitude and said alternating current signal having a predetermined frequency and a predetermined current amplitude, thereby forming a direct voltage drop signal and an alternating voltage drop signal across said resistor; PA1 means for chopping said direct voltage drop signal at a given frequency and filtering said chopped signal at the same frequency thereby forming a first output alternating signal; PA1 means for amplifying said alternating voltage drop signal, thereby forming a second output alternating signal, said amplifying means being controlled by said third switching means whereby the amplification factor of the alternating voltage drop signal is equal to the attenuation factor of the selected alternating voltage signal; PA1 a voltage meter; and PA1 means for selectively applying to said voltage meter the first and second output alternating signals.
Most alternating current apparatus carry out the resistor measurement at a frequency equal to 1000 Hz. Some of them can be connected to an auxiliary adjustable frequency generator.
In these known measuring apparatus, the amplitude of the current signal which flows through the resistor to be measured is determined with low accuracy or else cannot be adjusted. For instance, if resistance values of relay contact blades are measured, these apparatus do not hold the relays in the true operating conditions that exist in telephone exchanges.